A time-of-flight mass spectrometer (which will be called “TOFMS” hereinafter) generally introduces accelerated ions into a flight space where neither electric field nor magnetic field is present, and separates a variety of ions into every mass (mass-to-charge ratio m/z to be exact) in accordance with the flight time for an ion to reach an ion detector. A TOFMS which utilizes an ion trap as the ion source is conventionally known and called an ion trap time-of-flight mass spectrometer (IT-TOFMS).
As illustrated in FIG. 1, a typical ion trap 2 is what is called a three-dimensional quadrupole type, and is composed of a substantially annular ring electrode 21 and a pair of end cap electrodes 22 and 23 provided on both sides of the ring electrode 21. Generally, a radio-frequency voltage is applied to the ring electrode 21 to form a quadrupole electric field in an ion trap space inside the ion trap 2 so that ions are captured and stored by the electric field. In one case, ions are created outside of the ion trap 2 and then introduced into the ion trap 2, while in the other case, ions are created inside the ion trap 2. The theoretical explanation of the ion trap 2 is described in detail in Non-Patent Document 1 or other documents.
In performing a mass analysis with an IT-TOFMS, the application of the radio-frequency voltage to the ring electrode 21 is halted at the point in time when the ions to be analyzed are prepared inside the ion trap 2 by a series of processes as previously described. Almost at the same time with or somewhat later than the halt, a voltage for expelling ions is applied between the pair of end cap electrodes 22 and 23 in order to form an ion-expelling electric field inside the ion trap 2. Ions are accelerated by this electric field, ejected from the ion trap 2 through an exit aperture 25, and introduced into a time-of-flight mass analyzer 3 provided outside of the ion trap 2, to perform a mass analysis.
In the state where ions are captured in the ion trap 2, the ions are repeatedly accelerated and decelerated by the radio-frequency electric field. Therefore, before making the ions exit from the ion trap 2, it is common to gradually diminish the radio-frequency voltage's amplitude in order to decrease the velocity spread of the ions for the sake of the improvement of mass resolution and mass accuracy. However, this weakens the capturing action by the radio-frequency electric field, which results in a spatial spread of the ions. Consequently, the loss of ions in passing through the exit aperture 25 increases, which leads to a decrease of the detection sensitivity in the time-of-flight mass analyzer 3.
Since the aforementioned acceleration and deceleration of the ions in the ion trap 2 are synchronized with the alternation of the ion-capturing radio-frequency electric field, if it is possible to halt the ion-capturing radio-frequency electric field at a phase at which the ions' kinetic energy is minimized, the mass resolution and mass accuracy can be increased without decreasing the detection sensitivity. However, a conventional and general analog ion trap uses an inductance-capacitance (LC) resonator to apply an ion-capturing radio-frequency voltage, and such a circuit has a disadvantage in that it is difficult to quickly halt the voltage application at a desired phase. Given this factor, in the ion trap apparatus described in Patent Document 1, ions are expelled from the ion trap 2 with a relatively small spatial spread of the ions, using a characteristic phenomenon whereby an operation of halting the application of the ion-capturing radio-frequency voltage at a specific phase makes the ring electrode's electric potential to be at a predetermined value after a certain period of time regardless of the immediately preceding amplitude.
In practice, due to the use of a resonator for a voltage generation circuit, the voltage being applied to the ring electrode will remain for some time even after the operation of halting the application of the ion-capturing radio-frequency voltage is performed. Accordingly, the velocity spread of the ions when the ions are expelled might increase, due to the effect of the electric field remaining in the ion trap after the point in time when the operation of halting the application of the ion-capturing radio-frequency voltage is performed and before ions are actually expelled from the ion trap. This might decrease the mass resolution and mass accuracy.
In the meantime, digital ion traps in which a rectangular-wave radio-frequency voltage is applied to the ring electrode have recently been developed (refer to Patent Document 2, Non-Patent Document 2 and other documents for example) as an alternative to the previously described analog ion trap using a resonator. In a digital ion trap, it is possible to perform the mass selection of an ion to be stored, by changing the wavelength of the rectangular-wave radio-frequency voltage while maintaining its amplitude. In a voltage generation circuit of such a digital ion trap, a rectangular-wave voltage is generated by changing, with switches, a direct current voltage generated in a direct current power source as described in Patent Document 2 for example. According to this method, it is possible, in principle, to halt the application of voltage at a desired timing.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-214077    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-512702    [Non-Patent Document 1] R. E. March and R. J. Hughes, Quadrupole Storage Mass Spectrometry, John Wiley & Sons, 1989, pp. 31-110.    [Non-Patent Document 2] Furuhashi et al., “Development of Digital Ion Trap Mass Spectrometer,” Shimadzu Review, Shimadzu Review Editor, vol. 62, no. 3-4, pp. 141-151, Mar. 31, 2006.